Finely, divided, spherical, two-layer solid particles

ABSTRACT

Finely divided, spherical, two-layer solid particles having a diameter of from 5 to 100 nm and consisting of 
     (A) from 60 to 70% by weight of a core of spherical, rhombohedral α-Fe 2  O 3  having a purity greater than 98% and 
     (B) from 30 to 40% by weight of a shell of a basic metal hydroxide sulfate which has an alkali metal ion content of less than 500 ppm and is of the general formula 
     
         M.sub.v Mn.sub.w Zn.sub.x (OH).sub.y (SO.sub.4).sub.z 
    
      where M is Mg, Co, Ni, Cd, divalent Fe and/or divalent Cu, v is from O to 0.8, w is from 0.1 to 0.9, x is from 0.1 to 0.9, y is from 1.01 to 1.99, z is from 0.005 to 0.505, v++x 1 and 0.5 y+z=1, and a process for the production of finely divided, magnetic, cubic or hexagonal ferrite particles.

The present invention relates to novel finely divided, spherical,two-layer solid particles F₁ having a diameter of from 5 to 100 nm andconsisting of

(A) from 60 to 70% by weight of a core of spherical, rhombohedral α-Fe₂O₃ having a purity greater than 98% and

(B) from 30 to 40% by weight of a shell of a basic metal hydroxidesulfate which has an alkali metal ion content of less than 500 ppm andis of the general formula I

    M.sub.v Mn.sub.w Zn.sub.x (OH).sub.y (SO.sub.4).sub.z      I

where M is Mg, Co, Ni, Cd, divalent Fe and/or divalent Cu, v is from 0to 0.8, w is from 0.1 to 0.9, x is from 0.1 to 0.9, y is from 1.01 to1.99, z is from 0.005 to 0.505, v+w+x=1 and 0.5 y+z =1.

The present invention furthermore relates to an improved process for theproduction of finely divided, magnetic, cubic or hexagonal ferriteparticles F₁ ' or F₂ ', in which finely divided, two-layer solidparticles F₁ or F₂ consisting of an iron oxide or iron oxide hydroxidecore (A) and a metal hydroxide-containing shell (B) are formed asintermediates.

Solid particles are referred to as spherical when they are round, almostround, cuboidal or octahedral; they are referred to as acicular whentheir ratio of length to thickness is greater than 6:1. Finely dividedsolid particles are particles having a maximum diameter of from 5 to2000 nm. Assuming that the solid particles are monodisperse andsubstantially pore-free, this diameter roughly corresponds to an innersurface area determined according to Brunauer, Emmet and Teller (BETsurface area) of from 0.2 to 30 m² /g.

Magnetic substances which become highly magnetic as a result of anexternally applied magnetic field and have a small residualmagnetization or remanence in the absence of an external magnetic fieldare referred to as magnetically soft, whereas those having a highremanence are referred to as magnetically hard.

Ferrites are ceramic iron oxide compounds. Cubic ferrites have theapproximate composition MFe₂ O₄ (where M is a divalent metal ion),possess a spinel structure and predominantly tend to form sphericalparticles. The composition of hexagonal ferrites cannot be expressed ina simple manner since their stoichiometry and crystal structure varygreatly. Hexagonal ferrites predominantly tend to form anisotropicparticles, such as platelets or needles.

Magnetically hard recording media are the known audio tapes, videotapes, storage disks, floppy disks and magnetic stripes on, for example,check cards or credit cards, based on iron oxide.

Plastics which contain large amounts of finely divided, magneticparticles as fillers are referred to as plastoferrite materials.

Finely divided, acicular, two-layer solid particles F₂ which consist ofan acicular core (A) of iron oxide or iron oxide hydroxide and a shell(B) containing manganese hydroxide and zinc hydroxide are disclosed inGerman Laid-Open Application DOS No. 2,916,403. They are, however,unsuitable for the production of shaped articles from magnetically softferrites, such as cores or transformer cores.

It is known that such acicular solid particles F₂ are produced byprecipitating metal hydroxides from their aqueous salt solutions bymeans of an aqueous base onto the surface of finely divided, aciculariron oxide or iron oxide hydroxide particles dispersed in water, afterwhich the resulting two-layer particles F₂ are isolated, washed withwater and then dried. Thereafter, the particles F₂ are sintered at from800° to 1100° C., which is generally referred to as green firing. Thisgives finely divided, acicular, magnetically hard hexagonal ferriteparticles F₂ which can be used for the production of magnetically hardrecording media and of plastoferrite materials. The disadvantage here isthat, when the particles F₂ are washed with water, the composition oftheir shell (B) alters in an undesirable manner, which substantiallyimpairs the reproducibility of the process and may considerably reducethe quality of the ferrite particles F₂, so that they are no longersuitable for the above-mentioned purposes.

Finely divided, spherical, magnetic, cubic manganese zinc ferriteparticles F₁ " are usually produced by simultaneous precipitation of thehydroxides from metal salt solutions having appropriate compositions, bymeans of aqueous bases, such as ammonia, sodium hydroxide solutionand/or sodium carbonate (cf. U.S. Pat. Nos. 4,097,392 and 3,822,210).The resulting single-layer, finely divided solid particles containingmetal hydroxides (hydroxide particles) are filtered off, washed withwater, dried, and sintered at from 500° to 800° C. Particle growthresults in the formation of the desired finely divided, spherical,magnetically soft, cubic ferrite particles F₁ 41 . These are granulatedand screened in order to obtain a very narrow particle sizedistribution. Thereafter, additives, such as organic binders, are added,and the resulting mixture is converted, under high pressure, to thegreen compact of the shaped article to be produced. This green compactis then fired at from 1000° to 1200° C. to give the shaped article, forexample a core (cf. U.S. Pat. No. 4,097,392). The disadvantage here isthat the hydroxide particles are difficult to filter off, and washingwith water frequently results in an undesirable change in the particlecomposition, which substantially impairs the reproducibility of theprocess and considerably reduces the quality of the ceramic shapedarticles.

It is an object of the present invention to provide finely divided,spherical, two-layer solid particles F₁ which are particularly suitablefor the production of finely divided, spherical, magnetically soft,cubic ferrite particles F₁ '.

It is a further object of the invention to provide a novel improvedprocess for the production of finely divided, magnetic, cubic orhexagonal ferrite particles F₁ ' or F₂ ', in which finely divided,two-layer solid particles F₁ or F₂ consisting of an iron oxide or ironoxide hydroxide core (A) and a metal hydroxide-containing shell (B) areformed as intermediates, and which gives ferrite particles F₁ ' or F₂ 'which, with regard to their narrow particle size distribution, theirpurity, their magnetic properties and their processing properties, aresuperior to the prior art, so that they are particularly suitable forthe production of magnetically soft, ceramic shaped articles or ofmagnetically hard recording media and of plastoferrite materials.

We have found that this object is achieved by the solid particles F₁defined at the outset.

We have furthermore found a process for the production of finelydivided, magnetic, cubic or hexagonal ferrite particles F₁ ' or F₂ ' by

(a) precipitating metal hydroxides from their aqueous salt solutions bymeans of an aqueous base onto the surface of finely divided iron oxideor iron oxide hydroxide particles dispersed in water, with the formationof finely divided, two-layer solid particles F₁ or F₂ consisting of acore (A) and a shell (B) and

(b) isolating, washing, drying and sintering (green firing) thetwo-layer solid particles F₁ or F₂,

wherein from 5 to 20% by volume, based on the volume of the dispersion,of a C₁ -C₆ -alcohol are added to the aqueous dispersion of theparticles F₁ or F₂ before the latter are isolated, after which theparticles are isolated and then washed several times with a mixture offrom 0.05 to 0.3 part by volume of a C₁ -C₆ -alcohol and 1 part byvolume of water.

The novel particles F₁ have a diameter of from 5 to 100 advantageouslyfrom 8 to 90, in particular from 10 to 80, nm. They consist of a core(A) and a shell (B) which surrounds this core (A).

The proportion of the cores (A) in the particles F₁ is from 60 to 70,preferably from 62 to 68, in particular from 64 to 66%, by weight. Thecores (A) consist of spherical rhombohedral α-Fe₂ O₃ having a puritygreater than 98%, preferably 99%, in particular 99.4%. According to theinvention, carbonyl iron oxide which is prepared from iron pentacarbonyland has a purity greater than 99.4%, in particular greater than 99.5% orhigher, is particularly advantageous here. α-Fe₂ O₃ particles which aresuitable according to the invention have a diameter of from 10 to 80 nmand a BET surface area of from 10 to 25 m² /g.

The proportion of the shells (B) in the particles F₁ is from 30 to 70,preferably from 32 to 38, in particular from 34 to 36%, by weight. Theshells contain less than 500 ppm of alkali metal ions and consist ofbasic metal hydroxide sulfates. The term basic indicates that, in theshells, the number of equivalents of hydroxyl anions is always greaterthan the number of equivalents of sulfate anions, the number ofequivalents being the number of negative charges required toelectrically neutralize the cations.

Examples of shells (B) which are suitable according to the invention andare of the general formula I, where v≠0, are:

Mg₀.1 Mn₀.6 Zn₀.3 (OH)₁.4 (SO₄)₀.3,

Ni₀.3 Mn₀.2 Zn₀.5 (OH)₁.8 (SO₄)₀.1,

Cd₀.05 Mn₀.65 Zn₀.3 (OH)₁.08 (SO₄)₀.46,

divalent Fe₀.8 Mn₀.1 Zn₀.1 (OH)₁.9 (SO₄)₀.05,

divalent Cu₀.02 Mn₀.7 Zn₀.28 (OH)₁.4 (SO₄)₀.3,

Mg₀.2 Co₀.1 Mn₀.6 Zn₀.1 (OH)₁.7 (SO₄)₀.15,

Co₀.1 Ni₀.15 Mn₀.5 Zn₀.25 (OH)₁.3 (SO₄)₀.35,

divalent Fe₀.7 divalent Cu₀.02 Mn₀.14 Zn₀.14 (OH)₁.8 (SO₄)₀.1,

Mg₀.01 Co₀.01 Ni₀.01 Mn₀.67 Zn₀.3 (OH)₁.3 (SO₄)₀.35 and

Co₀.01 Ni₀.01 Cd₀.01 Mn₀.7 Zn₀.27 (OH)₁.85 (SO₄)₀.075.

Examples of shells (B) which are suitable according to the invention andof the general formula I, where v=0, are:

Mn₀.7 Zn₀.3 (OH)₁.02 (SO₄)₀.49,

Mn₀.1 Zn₀.9 (OH)₁.98 (SO₄)₀.01,

Mn₀.9 Zn₀.1 (OH)₁.5 (SO₄)₀.25 and

Mn₀.5 Zn₀.5 (OH)₁.6 (SO₄)₀.2.

For shells (B) which are particularly suitable according to theinvention and of the general formula I, v=0, w=from 0.6 to 0.8, x=from0.2 to 0.4, y=from 1.4 to 1.9 and z=from 0.05 to 0.3. Examples of suchshells (B) are:

Mn₀.6 Zn₀.4 (OH)₁.4 (SO₄)₀.3,

Mn₀.7 Zn₀.3 (OH)₁.9 (SO₄)₀.05,

Mn₀.65 Zn₀.35 (OH)₁.5 (SO₄)₀.25,

Mn₀.72 Zn₀.28 (OH)₁.76 (SO₄)₀.12 and

Mn₀.72 Zn₀.28 (OH)₁.64 (SO₄)₀.18.

Particularly advantageous novel particles F₁ have a diameter of from 10to 80 nm and a BET surface area of from 15 to 22 m² /g. Their cores (A)consist of carbonyl iron oxide having a purity of 99.5% or higher. Theproportion of cores (A) in the particles F₁ is from 64 to 66% by weight.Their shells (B) contain less than 500 ppm of alkali metal ions. Theyconsist of manganese zinc hydroxide sulfate having the particularlysuitable composition stated above. Very particularly advantageous novelparticles F₁ have a sulfate content of from 4 to 6% by weight, based ontheir total weight; this corresponds to the conditions y=from 1.64 to1.76 and z=from 0.12 to 0.18 in formula I.

The novel particles F₁ can be prepared by the prior art processes.Particularly suitable for this purpose is the process in which thedesired amount of finely divided α-Fe₂ O₃ particles (BET surface area:10-20 m² /g) is dispersed in water, and the appropriate salt mixtureshaving the desired composition are dissolved in the dispersion. Allsalts which are water-soluble, such as the chlorides, bromides,nitrates, sulfates or acetates, are suitable, the sulfates beingpreferred.

An aqueous base is then added to this aqueous mixture in an amount suchthat a pH of from 9 to 11 is obtained. Sodium hydroxide solution orpotassium hydroxide solution is suitable, the former being preferred. Ingeneral, the mixture is stirred for a relatively long time, the pH beingkept constant at 9-11, in particular 10.

The resulting dispersion of the particles F1 is then generally filtered,after which the particles are washed with water and dried.

Where divalent iron is used, it is advisable to carry out the processsteps under an inert gas.

According to the invention, it is very particularly advantageous if theparticles F₁ are prepared and isolated in the novel process for theproduction of ferrite particles F₁ ' or F₂ '.

However, the novel process is not restricted merely to the production ofparticles F1 but can also very successfully be used for the synthesis ofthe particles F₂ which are not according to the invention. Furthermore,it may be employed for the production of ferrite particles F₁ ' or F₂ '.

From the point of view of process engineering, the novel process has nospecial features, i.e. in principle no specially developed and adaptedapparatuses are required to carry out the process, and each individualprocess step is based on a conventional chemical method.

The novel process starts from an aqueous dispersion of finely dividediron oxide or iron oxide hydroxide particles, the amount of theseparticles depending on the desired stoichiometric composition of the endproducts. Examples of suitable finely divided iron oxide or iron oxidehydroxide particles are finely divided particles of acicular γ-Fe₂ O₃ orα-FeO(OH) (Goethite) or of spherical α-Fe₂ O₃.

The appropriate metal salts are dissolved in the dispersion. The typeand amount of each of the metal salts depend on the desiredstoichiometric composition of the end product.

Examples of suitable salts are those mentioned above, of which thesulfates are preferred.

A 5-10 N alkali metal hydroxide solution is addcd to this mixture atroom temperature while stirring, until a pH of from 9 to 11 is obtained.The duration of the addition is chosen so that a mechanically andchemically stable shell (B) can form on the core (A). In general, thisduration is from 10 minutes to 2 hours, depending on the size of thebatch.

The dispersion of the particles F₁ or F₂ is then stirred at roomtemperature for a further 10 minutes to 1 hour, and, if necessary,further alkali metal hydroxide solution is added in order to keep the pHof the dispersion at 9-11, in particular 10.

In the procedure according to the invention, from 5 to 20% by volume,based on the volume of the dispersion, of a C₁ -C₆ -alcohol are thenadded to this dispersion, while stirring.

Examples of suitable alcohols are ethanol, n-propanol, isopropanol,n-butanol, isobutanol, tert-butanol, n-pentanol, n-hexanol andcyclohexanol as well as mixtures of these, isopropanol beingadvantageous.

After the particles F₁ or F₂ have been isolated, they are washed severaltimes with a mixture of 1 part by volume of water and from 0.05 to 0.3part by volume of the abovementioned alcohols, by the procedureaccording to the invention. Washing is continued until the water/alcoholmixture gives a neutral reaction and anions and/or cations are no longerdetectable therein, i.e. conventional detection reactions for therelevant ions are negative.

The particles are then dried.

These process steps can, if necessary, be carried out under an inertgas.

The particles thus obtained are either the novel solid particles F₁ orsolid particles F₂ which are known per se.

In the further course of the process, the particles F₁ are converted bysintering, i.e. by green firing, at from 800° to 1100° C. into ferriteparticles F₁ ' having a composition which is known per se. The sinteringtime is in general from 30 minutes to 2 hours. During sintering, thenumber of particles decreases and the particle diameter increases to100-2000 nm, depending on the sintering time and sintering temperature.The sulfate content decreases dramatically and reaches values of lessthan 0.01% by weight. The atomic ratio of the metals present in theferrite particles F₁ ' corresponds to that of the starting compounds.Where deviations from this occur, they are no greater than ±3%. Thealkali metal ion content of the ferrite particles F₁ ' is less than 500ppm.

In the further course of the process, the conventional solid particlesF₂ are converted by sintering at from 800° to 1100° C. into ferriteparticles F₂ ' having a composition which is known per se. The sinteringtime is in general from 30 minutes to 2 hours. The sintering results inonly a slight change in the particle size. The sulfate content decreasesdramatically and reaches values of less than 0.01% by weight. The atomicratio of the metals present in the ferrite particles F₂ corresponds tothat of the starting compounds. Where deviations from this occur, theyare no greater than ±3%. The alkali metal ion content of the ferriteparticles F₂ ' is less than 500 ppm.

The ferrite particles F₁ ' produced by the novel process can beconverted to magnetically soft, ceramic shaped articles in aconventional manner.

The ferrite particles F₂ ' produced by the novel process can be used forthe production of magnetically hard recording media and of plastoferritematerials.

The novel solid particles F₁ have a large number of special advantagesover the prior art. For example, they are particularly pure and for thisreason alone are very suitable for the production of magnetically softferrites F₁ ' and magnetically soft, ceramic shaped articles. They canbe produced in an exactly reproducible manner and with a very narrowparticle size distribution. Their shells are mechanically and chemicallyvery stable, so that the particles suffer no damage during furtherhandling and/or during storage.

The novel process also has particular advantages over the prior art. Forexample, this process can be used to produce finely divided, magnetic,cubic or hexagonal ferrite particles F₁ ' or F₂ ' in an exactlyreproducible manner and in high purity, the particularly low alkalimetal ion content being especially noteworthy. This particularly lowalkali metal ion content is obtained without having to accept anundesirable change in the particle composition. The cubic or hexagonalferrite particles F₁ ' or F₂ ' produced by the novel process thereforegive particularly advantageous magnetically soft shaped articles ormagnetically hard recording media and plastoferrite materials.

The novel process has very particular advantages over the prior art whenthe said process is used for the production of the novel solid particlesF₁ and for further processing these particles to give ferrite particlesF₁ '. This gives ferrite particles F₁ ' whose composition, low alkalimetal ion content and narrow particle size distribution are exactlyreproducible. Moreover, the ferrite particles F₁ ' suffer no damageeither during transport or during prolonged storage and furtherprocessing. In addition, the sintering can be carried out at acomparatively low temperature in a comparatively short time.

EXAMPLES

In the Examples and Comparative Experiments, the particle compositionwas determined from the increase in weight of the dispersed iron oxideor iron oxide hydroxide particles and chemical elemental analysis. Thediameter of the particles was measured with the aid of the transmissionelectron microscope. The inner surface area of the particles wasdetermined by the BET method.

EXAMPLE 1 Production of solid particles F₁ and of ferrite particles F₁ 'by the novel process

A mixture calculated to give a metal atom ratio in the ferrite of 2.6Mn:1Zn:8.4Fe (=Mn₀.65 Zn₀.25 Fe₂.1 O₄) and consisting of 143.7 g ofα-Fe₂ O₃ (red carbonyl iron oxide, purity 99.5%; BET surface area 10-20m² /g; diameter of the spherical particles 8-70 nm), 94.2 g of MnSO₄.H₂O, 61.6 g of ZnSO₄.7H₂ O and 1200 ml of water was initially taken underan inert gas.

160 ml of 8 N sodium hydroxide solution were added to this mixture inthe course of 15 minutes at room temperature and while stirring, until apH of 10 was obtained. The reaction mixture was stirred for a further 15minutes at room temperature, the pH being kept constant at 10 by addingfurther 8 N sodium hydroxide solution.

120 ml of isopropanol were added to the resulting dispersion of thesolid particles F1. Thereafter, the particles were filtered off andwashed with eight times 1 liter of a mixture of water and isopropanol(volume ratio 10:1) and then dried and divided into portions.

One portion of the resulting solid particles F₁ was analyzed, andanother was sintered for 60 minutes at 1000° C. and then investigated.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the required ferrite composition can very readilybe obtained with the aid of the novel process, a very low sodium ioncontent being achieved without a loss of manganese and/or zinc having tobe accepted; this is a fundamental advantage for the further processingof the particles F₁ and of the ferrite particles F₁ ' produced fromthem. Moreover, the ferrite particles F₁ ' had a narrow particle sizedistribution and a particularly low remanence and were spherical inshape. They were very suitable for the production of magnetically soft,ceramic shaped articles.

EXAMPLES 2 AND 3 Preparation of solid particles F₁ and ferrite particlesF₁ ' by the novel process Preparation method

Example 1 was repeated twice, except that in Example 2, the salts andα-Fe₂ O₃ were initially taken in 800 ml instead of 1200 ml of water, andthe solid particles F₁ were washed with ten times 1 liter of a 10:1water/isopropanol mixture, and in Example 3, 300 ml instead of 120 ml ofisopropanol were added to the suspension of the solid particles F₁, andthe solid particles I were washed with six times 1 liter of a 4:1water/isopropanol mixture.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the desired ferrite composition can be excellentlyreproduced using the novel process, the particular advantages stated inExample 1 always being obtained.

EXAMPLES 4 AND 5 Preparation of solid particles F₂ and of ferriteparticles F₂ ' by the novel process Preparation method

Example 1 was repeated twice, except that acicular α-FeO(OH) having aBET surface area of 27 m² /g and a ratio of particle length to particlethickness of 10 was used.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the desired ferrite composition can be obtainedand reproduced in an excellent manner using the novel process, aparticularly low sodium ion content also being obtained without a lossof manganese and/or zinc having to be accepted; this is a fundamentaladvantage. Moreover, the ferrite particles F₂ ' have a uniform, wellpronounced acicular shape. They are very useful for the production ofmagnetically hard recording media and of plastoferrite materials.

EXAMPLE 6 Preparation of solid particles F₁ and of ferrite particles F₁' by a conventional process Preparation method

Example 1 was repeated, except that the solid particles F₁ were washedwith four times 1 liter of water.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the composition of the solid particles F₁ producedby the conventional process differed somewhat from the desiredcomposition, although an advantageously low sodium ion content wasobtained, which was accompanied by a certain loss of manganese and zinc.The particles F₁ were spherical in shape. The composition of the ferriteparticles F₁ ' produced from these particles likewise differed slightlyfrom the desired composition. The BET surface area of the ferriteparticles F₁ ' was comparatively high and its particle size distributionwas broad. The particles were predominantly spherical in shape. Theywere suitable for the production of magnetically soft, ceramic shapedarticles.

COMPARATIVE EXPERIMENT 1 Production of ferrite particles F₁ " by aconventional process Preparation method

A mixture calculated to correspond to an empirical formula Mn₀.65 Zn₀.25Fe₂.1 O₄ and consisting of 486.5 g of FeCl₃.6H₂ O, 110.3 g of MnCl₂.4H₂O, 29.2 g of ZnCl₂ and 1200 ml of H₂ O was initially taken in air.

872 ml of 8 N sodium hydroxide solution were added to this solution inthe course of 20 minutes while stirring, so that a pH of 10 wasobtained. The reaction mixture was stirred for a further 15 minutes atroom temperature, the pH being kept constant at 10 by adding further 8 Nsodium hydroxide solution, and oxygen being passed through the reactionmixture. The resulting dispersion of finely divided mixed manganese zinciron hydroxides (hydroxide particles) was filtered, the filtration beingdifficult because particularly finely divided particles either passedthrough the filter or, when a fine-pore filter was used, blocked thelatter.

The hydroxide particles filtered off were washed with ten times 1 literof water and then dried and divided into portions.

One of the portions was analyzed and the other was sintered for 60minutes at 1000° C., after which the resulting ferrite particles F₁ "were investigated.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the hydroxide particles had a disadvantageouslyhigh sodium ion content and their composition differed substantiallyfrom the desired composition. Their particle size distribution wasbroad. Some of the particles had a poorly defined acicular shape whileanother, major proportion had an irregular shape. Well defined,spherical particles were in the minority. Furthermore, the ferriteparticles F₁ " did not have the desired composition and shape. Theirsodium content was disadvantageously high. They were suitable only to alimited extent for the production of magnetically soft, ceramic shapedarticles.

COMPARATIVE EXPERIMENTS 2 AND 3 Production of solid particles F₂ and offerrite particles F₂ ' by a conventional process Preparation method

Examples 4 and 5 were repeated, except that the solid particles F₂ werealways washed with four times 1 liter of water in this case.

The results of the chemical analysis and the determination of the BETsurface area and of the particle diameter are shown in the Table.

The results show that the known process was inferior to the novelprocess with regard to the exactness with which compositions of solidparticles and of ferrites could be obtained and reproduced. Furthermore,the acicular shape of the particles F₂ ' was not so well defined as inthe case of those produced by the procedure according to the invention,which was a disadvantage when they were used for the production ofmagnetically hard recording media and plastoferrite materials.

                                      TABLE                                       __________________________________________________________________________    Experimental Results - desired composition of the ferrite particles           F.sub.1 ', F.sub.2 ' or F.sub.1 ": Mn.sub.0.65 Zn.sub.0.25 Fe.sub.2.1         O.sub.4 ; metal atom ratio: 2.6 Mn:1 Zn:8.4 Fe                                                     Na.sup.⊕  content (in the                                                           SO.sub.4.sup.2⊖  content (in           Examples and         shell (B) of the solid                                                                  the shell (B) of the                                                                    Composition:                         Comparative                                                                          BET surface                                                                          Particle                                                                             particles or in the                                                                     solid particles or in                                                                   empirical formula                    Experiments                                                                          area   diameter                                                                             ferrite particles)                                                                      the ferrite particles)                                                                  and metal atom                       No.    (m.sup.2 /g)                                                                         (nm)   (ppm)     (% by weight)                                                                           ratio                                __________________________________________________________________________    Example 1                                                                     Particles F.sub.1                                                                    20     10-80  Shell (B): 90                                                                           Shell (B): 4                                                                            Shell (B):                                                                    Mn.sub.0.72 Zn.sub.0.28 (OH).sub.                                             1.76 (SO.sub.4).sub.0.12                                                      2.57 Mn:1 Zn:8.13 Fe                 Ferrite                                                                              0.2      500-2,000                                                                          30        0.005     Mn.sub.0.659 Zn.sub.0.256                                                     Fe.sub.2.08 O.sub.4                  particles F.sub.1 '                      2.57 Mn:1 Zn:8.13 Fe                  Example 2                                                                    Particles F.sub.1                                                                    21     15-80  Shell (B): 90                                                                           Shell (B): 6                                                                            Shell (B):                                                                    Mn.sub.0.72 Zn.sub.0.28 (OH).sub.                                             1.64 (SO.sub.4).sub.0.18                                                      2.6 Mn:1 Zn:8.36 Fe                  Ferrite                                                                              0.25   1,000-2,000                                                                          30        0.001     Mn.sub.0.65 Zn.sub.0.25 Fe.sub.2.                                             1 O.sub.4                            particles F.sub.1 '                      2.6 Mn:1 Zn:8.4 Fe                   Example 3                                                                     Particles F.sub.1                                                                    19      8-90  Shell (B): 340                                                                          Shell (B): 5                                                                            Shell (B):                                                                    Mn.sub.0.73 Zn.sub.0.27 (OH).sub.                                             1.7 (SO.sub.4).sub.0.15                                                       2.67 Mn:1 Zn:8.36 Fe                 Ferrite                                                                              0.2    10-80  120       0.005     Mn.sub.0.67 Zn.sub.0.25 Fe.sub.2.                                             08 O.sub.4                           particles F.sub.1 '                      2.68 Mn:1 Zn:8.32 Fe                 Example 4                                                                     Particles F.sub.2                                                                    29     300-500                                                                              Shell (B): 100                                                                          Shell (B): 7                                                                            Shell (B):                                         (= length)                 Mn.sub.0.72 Zn.sub.0.28 (OH).sub.                                             1.56 (SO.sub.4).sub.0.22                                                      2.6 Mn:1 Zn:8.35 Fe                  Ferrite                                                                              7        500-1,100                                                                          35        0.001     Mn.sub.0.65 Zn.sub.0.25 Fe.sub.2.                                             1 O.sub.4                            particles F.sub.2                                                                           (= length)                 2.6 Mn:1 Zn:8.4 Fe                   Example 5                                                                     Particles F.sub.2                                                                    28     250-500                                                                              Shell (B): 90                                                                           Shell (B): 5                                                                            Shell (B):                                         (= length)                 Mn.sub.0.72 Zn.sub.0.28 (OH).sub.                                             1.7 (SO.sub.4).sub.0.15                                                       2.58 Mn:1 Zn:8.34 Fe                 Ferrite                                                                              8.5      450-1,000                                                                          30        0.004     Mn.sub.0.65 Zn.sub.0.25 Fe.sub.2.                                             1 O.sub.4                            particles     (= length)                 2.58 Mn:1 Zn:8.4 Fe                  Example 6                                                                     Particles F.sub.1                                                                    22     10-80  Shell (B): 100                                                                          Shell (B): 6                                                                            Shell (B):                                                                    Mn.sub.0.71 Zn.sub.0.29 (OH).sub.                                             1.6 (SO.sub.4).sub.0.2                                                        2.5 Mn:1 Zn:8.8 Fe                   Ferrite                                                                              0.1    1,000-2,000                                                                          35        0.001     Mn.sub.0.62 Zn.sub.0.24 Fe.sub.2.                                             14 O.sub.4                           particles F.sub.1 '                      2.58 Mn:1 Zn:8.9 Fe                  Comparison 1                                                                  Hydroxide                                                                            25      5-90  68,000    --        2.5 Mn:1 Zn:8.5 Fe                   particles                                                                     Ferrite                                                                              0.05   1,500-3,000                                                                          68,000    --        Mn.sub.0.66 Zn.sub.0.26 Fe.sub.2.                                             08 O.sub.4                           particles F.sub.1 "                      2.5 Mn:1 Zn:8.0 Fe                    Comparison 2                                                                 Particles F.sub.2                                                                    25       400-1,000                                                                          Shell (B): 200                                                                          Shell (B): 5                                                                            Shell (B):                                         (= length)                 Mn.sub.0.74 Zn.sub.0.26 (OH).sub.                                             1.7 (SO.sub.4).sub.0.15                                                       3 Mn:1 Zn:11.3 Fe                    Ferrite                                                                              7        500-1,300                                                                          70        0.001     Mn.sub.0.58 Zn.sub.0.2 Fe.sub.2.2                                             2 O.sub.4                            particles F.sub.2 '                                                                         (= length)                 2.9 Mn:1 Zn:11.1 Fe                  Comparison 3                                                                  Particles F.sub.2                                                                    28     300-900                                                                              Shell (B): 110                                                                          Shell (B): 4                                                                            Shell (B):                                         (= length)                 Mn.sub.0.77 Zn.sub.0.23 (OH).sub.                                             1.76 (SO.sub.4).sub.0.12                                                      3.3 Mn:1 Zn:12.3 Fe                  Ferrite                                                                              7.2      500-1,200                                                                          40        0.003     Mn.sub.0.6 Zn.sub.0.18 Fe.sub.2.2                                             2 O.sub.4                            particles F.sub.2 '                                                                         (= length)                 3.3 Mn:1 Zn:12.3                     __________________________________________________________________________                                             Fe                               

I claim:
 1. Finely divided, spherical, two-layer solid particles F₁ having a diameter of from 5 to 100 nm and consisting of(A) from 60 to 70% by weight of a core of spherical, rhombohederal α-Fe₂ O₃ having a purity greater than 98% and (B) from 30 to 40% by weight of a shell of a basic metal hydroxide sulfate which has an alkali metal ion content of less than 500 ppm and is of the formula I

    M.sub.v Mn.sub.w Zn.sub.x (OH).sub.y (SO.sub.4).sub.z      I

wherein M is Mg, Co, Ni, Cd, divalent Fe and/or divalent Cu, v is from 0 to 0.8, w is from 0.1 to 0.9, x is from 0.1 to 0.9, y is from 1.01 to 1.99, z is from 0.005 to 0.505, v+w+x=1 and 0.5 y+z=1. 